The invention is directed to a method for setting the idle distance of an engraving stylus for engraving a rotogravure printing cylinder from the cylinder surface of the rotogravure printing cylinder upon involvement of a sliding foot that is provided for the guidance of the engraving stylus connected to the sliding foot along the cylinder surface during engraving.
The invention is also directed to an adjustment mechanism, preferably for implementation of the method.
For rotogravure, a printing form is usually engraved into the cylinder surface of a rotogravure printing cylinder with an engraving stylus or with a plurality of engraving styli. The tip of the engraving stylus thereby normally comprises diamond.
For the engraving, the engraving stylus is driven in vibrating fashion with a prescribed frequency and amplitude. The image information to be engraved is superimposed as a signal on this basic vibration, so that the engraving stylus, when vibrating, enters into the cylinder surface with a different depth or does not enter thereinto according to the image information. Simultaneously, the rotogravure printing cylinder is driven in rotating fashion, and the engraving stylus is moved along the cylinder surface in the axial direction either continuously or in steps in order to achieve a continuation of the engraving image, which usually comprises a matrix of cups. During its relative motion over the cylinder surface, an engraving head comprising the engraving stylus is supported on the cylinder surface via a sliding foot, likewise preferably comprising diamond. For varying the spacing of the engraving head from the cylinder surface, the sliding foot can usually be retracted or extended farther transverse relative to the axis of the rotogravure form cylinder with a spindle, whereby “transverse” does not necessarily mean perpendicular to the axis and the sliding foot also need not necessarily lie in a radial path relative to the axis but, for example, can also be secantially aligned.
When the vibratory drive is turned off, the engraving stylus is situated at a slight clearance from the cylinder surface in its idle position, whereby this idle distance is prescribed by the setting of the extended length of the sliding foot. This idle distance, of course, must agree very precisely so that the image information is engraved into the cylinder surface as intended. The position of the engraving tip corresponds to the zero-axis crossing of the amplitude of the basic vibration of the engraving stylus without image information or given the image information, to not engrave at the corresponding location of the cylinder surface. The idle distance amounts, for example, to 3 through 5 micrometers, preferably 4 micrometers, and should be set to a precision of at least 10%.
During engraving, however, the engraving stylus wears over time or can even be damaged. The engraving styli must therefore be replaced by new ones relatively often. After such a replacement, however, the idle distance must be reset because, although the engraving tips or the cutting geometry of the engraving styli coincide, the engraving styli do not otherwise have structural lengths that coincide precisely enough. Just like the setting of the idle distance, thus the change of the engraving styli must be undertaken more frequently in engraving operations, namely more or less at the same time for a plurality of engraving machines since the rotogravure printing cylinders for the various colors of a color printing are usually engraved roughly simultaneously.
Up to now, one proceeded for setting the idle distance such that the new engraving stylus is roughly positioned and the spindle of the sliding foot is then moved slowly inward given a rotating rotogravure printing cylinder until the engraving stylus just barely lies against the cylinder surface and leaves a very slight scratch thereon, this lie there against being recognizable therefrom. With the assistance of a precision thread, the spindle is in turn moved outward by precisely the desired idle distance, as a result whereof this idle distance of the engraving stylus is directly produced.
This, however, is an inadequate and unsatisfactory procedure. Great care must be exercised in the described setting. A specific test cylinder is employed, this having to be chucked into the engraving machine for the setting and in turn removed therefrom later. Despite its high capital costs, the engraving machine cannot be used for engraving for the entirety of the adjustment work. The test cylinder does not rotate under machined drive, but is carefully manually turned a little over and over again by a person. The creation of the scratch is subjectively checked, and the return turning of the spindle of the sliding foot occurs with an angle prescribed by the operator. The procedure can therefore only be imprecisely objectively reproduced. Each operator obtains a somewhat different result.